Fiber-reinforced composite materials composed reinforcing fibers such as carbon fibers or glass fibers and a thermosetting resin such as an epoxy resin or phenol resin are light in weight and excellent in mechanical properties such as strength and rigidity, heat resistance and corrosion resistance, and therefore have been used in many fields such as aircraft, spacecraft, motor vehicles, rolling stock, ships, civil engineering & architecture and sporting goods. Especially for uses requiring high performance, fiber-reinforced composite materials using continuous reinforcing fibers are used. As reinforcing fibers, carbon fibers excellent in specific strength and specific modulus are popularly used, and as matrix resins, epoxy resins excellent in mechanical properties and adhesion to carbon fibers are popularly used.
Fiber-reinforced composite materials are produced by various methods. Resin transfer molding (hereinafter abbreviated as RTM) in which a liquid thermosetting resin, composition is injected into a reinforcing-fiber base material disposed in a mold and heated and cured to obtain a fiber-reinforced composite material attracts attention in recent years as a molding method excellent in productivity and low in cost.
In the case where a fiber-reinforced composite material is produced by RTM, it is often practiced that a preform is prepared by processing a reinforcing-fiber base material into a shape close to that of a desired composite material and installed in a mold and that subsequently a liquid thermosetting resin is poured into the mold.
As methods for preparing a preform, several methods are known such as a method of preparing a three-dimensional braid from reinforcing fibers and a method of laminating and stitching layers formed of a reinforcing-fiber woven fabric. A known very universal method is to laminate and shape layers formed of a sheet-like base material such as a reinforcing-fiber woven fabric using a hot-melt binder (also called a tackifier).
As the hot-melt binder, a resin composition that is not adhesive at room temperature but is softened to be adhesive at a high temperature is used. As the hot-melt binder, either of both a thermoplastic resin and a thermosetting resin can be used as described in patent document 1.
In the case where a thermoplastic resin is used as the hot-melt binder, since the glass transition temperature or melting point of the thermoplastic resin is relatively high, a very high temperature is required for thermally fusion-bonding the overlying regions adjacent to each other of a reinforcing-fiber base material. Consequently the productivity is low.
In the case where a thermosetting resin is used as the hot-melt binder, the binder per se can be of a type having curability (patent documents 2 to 4) or a type not having curability (patent documents 5 and 6). The former is excellent since it is curable irrespective of whether it is a liquid thermosetting resin, and the latter is excellent in storage stability.
On the other hand, a fiber-reinforced composite material containing a thermosetting resin such as an epoxy resin as a matrix resin has a problem that the impact resistance thereof declines, since the cured thermosetting resin is generally lower in fracture toughness than a thermoplastic resin. Especially since structural members of aircraft are required to be excellent in impact resistance against such impacts as the drop of a tool during assembling and hail during flying, the enhancement of impact resistance has been a large issue.
A fiber-reinforced composite material generally has a lamination structure, and if an impact acts on it, a high stress occurs between the respective layers, to cause cracking. For inhibiting cracking, it is effective to enhance the plastic deformability of the thermosetting resin, and as a means for it, mixing a thermoplastic resin with excellent plastic deformability is effective.
Various methods for mixing a thermoplastic resin have been examined using the prepreg method which is one of molding methods for producing fiber-reinforced composite materials. For example, there is a method of using a highly tough thermosetting resin enhanced in toughness by dissolving a thermoplastic resin into a thermosetting resin, as a matrix resin (see patent documents 7 and 8). However, if a thermoplastic resin is mixed with a thermosetting resin, the viscosity rises remarkably, and consequently there is a limit to the thermoplastic resin content, thus this method being not suitable especially for RTM in which the viscosity of the matrix resin is limited.
Further, as another means for enhancing the impact resistance, a method of making a thermoplastic resin or an elastomer exist between the respective layers where cracking is likely to occur. This method can be applied also to RTM by applying the aforementioned binder technique. In a fiber-reinforced composite material prepared by using a preform obtained by bonding the respective layers formed of a sheet-like base material such as a reinforcing-fiber woven fabric to each other by a binder, the binder component exists between the respective layers.
A thermoplastic resin has a problem of high processing temperature, since the glass transition temperature or melting point is high as described before. Accordingly, the processing temperature can be adjusted by mixing a thermosetting resin with the thermoplastic resin (patent documents 9 and 10). However, in the case where any of the binders described in these documents is used, the solubility of the binder may change due to the changes of molding conditions, especially the changes of temperature and heating rate, and it is difficult to exhibit the same toughness stably.